Photometry

Possible Applications in Environmental Analysis

Fig. 1: Diagram of the possibilities resulting from modular configurations of spectrometer components in connection with fibre optics. Instead of a cuvette holder for conventional cuvettes, the absorption test can be carried out in many other, almost freely selectable geometries.

Fig. 2: Function of pathlength cells which are based on the total reflection of the capillary filled with the sample of the solution. The pre-condition for this measuring configuration is that the refraction index of the capillary material is lower that of the inner core (liquid core waveguide principle).

Since Bouguer, Beer und Lambert laid down the theoretical basis for photometry 200 years ago and the fundamental experiments were carried out, the methods regarding instrumentation have changed significantly. Over the last three decades, the developments in optoelectronics and micro-technology in particular, have provided numerous impulses and thereby extended the possibilities of its application.

The devices available to the user of photometry today offer a diverse and multi-facetted spectrum, which ranges from the universally applicable multi-function devices through the simple photometer for on-site use, to analysers dedicated to specific applications. Besides the classic laboratory applications such as manual batch processes, photometry is used in connection with parallel or serial working batch analysers, as a reading appliance for micro-titre plate systems, as a detector for flow analysis processes (CFA, FIA) and in liquid chromatography and capillary electrophoresis, too.

Optical Configuration

Alongside the conventional photometer configurations with a broadband source of radiation, monochromator and light sensitive detector, there are diverse instrumental modifications available today. For example LEDs are used as a monochromatic source of radiation – most often for applications in mobile devices. By using diode-array-technology it is possible to record absorption spectra with single beam photometers over a wide range of the wavelength, and simultaneously transient signals, which appear in kinetic readings for example or in photometric detection in chromatography, too. Particularly interesting possibilities are made available by photometer systems supported by fibre optics, which allow the geometrical separation of the spectrometer from the location of the absorption measurement. For conventional spectrometers with a cuvette chamber, adaptors are available as accessories with which the light from the radiation source is fed to the measuring point with the aid of a fibre optic and from there it is returned to the detector. Completely modular systems have long been on offer by numerous companies (Fig 1.). These can be equipped with optional radiation sources (white light, UV-light LEDs or laser sources) and usually include a poly-chromator with an integrated CCD chip for the simultaneous logging of the whole spectrum in a wavelength range chosen by the user as a detector unit.

Cuvette Geometry

Most commercially available photometers are equipped with cuvette holders which can take rectangular cuvettes with a thickness of 1 cm (sometimes 5 cm and 10 cm). A wide variety of special cuvettes are on offer from manufacturers. For example some which are not as thick, for the analysis of highly concentrated solutions, for which only a very small amount of solution is needed (e.g. micro cuvettes with 5-10µl volume and 1cm thickness). Or cuvettes that have a U or Z shaped channel and are filled and emptied with the aid of a pump or in connection with a continuous flow analysis system (CFA, FIA or also HPLC and CE). Immersion probes, which can be placed directly in the solution, are also available in connection with photometers that are supported by fibre optics. A relatively new development is the so called long pathlength cell or liquid core waveguide. Here, the principle of total reflection on the internal wall of quartz capillaries or capillaries made of Teflon-PFA is used (see fig 2). This means that the optical pathlength of several metres can be achieved and, because of the low optical noise in the long pathlength, results in considerably improved detection limits and sensitivity. Besides the application of manual sample introduction with the aid of a syringe or dosage pump, pathlength cells can be advantageously integrated in flow systems [1,2].

Photometry in Environmental Analysis

One of the particular characteristics of photometry is the fact that, after appropriate derivatisation (in order to change the often few and / or only UV-absorbing analyte into a coloured compound with a high absorption co-efficient) several analytes can be determined in very different matrices. Not least for this reason, has photometry gained a strong position in the arsenal of analysis methods which are used for questions in environmental analysis, despite competition in particular, from ion-chromatography. So numerous photometry methods exist, especially in the area of anion analysis, which distinguish themselves in high selectivity and sensitivity as well as exhibiting a broad, dynamic measuring range. Possible interference is sufficiently well known due to long years of experience and measures to eliminate them are duly recorded in the literature [3-5]. Any adverse effects on the absorption testing caused by the sample matrix can often be compensated for by the appropriate adjustments in the standard solutions. The amount of work involved in preparing samples for photometry is generally much lower than for alternative methods like ion-chromatography as the tests themselves can be carried out in coloured or cloudy solutions – with the relevant referencing. Many photometric methods are included in diverse sets of standards for the analysis of water, sediment, soil, plant material and air samples (gases after absorption and dust after extraction) - not least due to the above mentioned characteristics. Over the decades they have proven themselves to be reliable in many laboratories and the accuracy of the results obtained with photometry have been confirmed in numerous comparative and co-operative tests.

As well as for ion determination in diverse environmental samples, photometry is also used for determining other parameters. Examples from the area of water analysis are the determining of colouring and cloudiness in water, the phenol index, from chlorophyll-A as well as photometric indications in titration for determining the acid and alkali capacity and the hardness of water.

An interesting aspect of photometry is the fact that it is very suitable for element speciation because of its selective derivatisation reactions. This means that in the cases of nitrogen and sulphur speciation for example, almost all occurring ionic forms in the various oxidation steps can be determined separately (usually without any influence from other species of the same element). And in metal speciation too, photometry offers diverse possibilities for differentiating between the various oxidation steps or chemical forms (e.g. Fe(II)/Fe(III), Cr(III)/Cr(VI), As(III)/As(V), free and complexly bound Al).

The conventional photometric determining procedures which are inadequately sensitive for certain questions, can be improved so much by the use of the above mentioned application of pathlength cuvettes sometimes, that new areas of application are being discovered. This instrumental variant has been accorded great interest in the area of ultra-trace detection of anions but also certain cations such as ammonium or iron (differentiated after the oxidation step) in extremely salty water such as sea water. Besides a considerable improvement in the detection limits, it is possible to obtain an interference-free determination in the presence of the high salt concentration matrix. With regards to sea water, ion-chromatography, usually considered competition to photometry for other samples, encounters difficulties which can only be overcome by using elaborate sample preparation techniques or so called cutting techniques (2D-IC).

But photometric detection has also gained a solid place in ion-chromatography, which usually uses conductivity detection (often connected in series and or in connection with post column derivatisation). In the case of determining some undesired by-products of water disinfection (bromate, chlorite, chlorate etc), the triiodide method [8] has proven to be particularly sensitive and in between times has found its way into the international (EPA) and national (DIN-ISO) sets of standards in water analysis. Another example of photometric detection in connection with IC is the post-column derivatisation with PAR for the trace determination of transition metals [9]. Here, the low selectivity of the colour reagent is an advantage as the chromatographically separated metal cations all absorb at the same wavelength and can therefore be detected one after the other with a fixed photometer setting. This method is definitely an attractive alternative to the more usual atom spectrometry techniques (AAS, ICP-OES/MS) and is comparably sensitive. That being said, preparing complex compound samples takes a lot more effort due to possible errors in the chromatographic separation and if there are big differences in the concentrations of the analyte there is a possibility of overlapping peaks. In my view, one of the many, particularly distinctive characteristics of photometry is the fact that numerous tests can be set up very easily and carried out with inexpensive devices. In connection with the so called cuvette tests, which include ready-made reagent packs, all the usual photometric determination methods in environmental analysis can often be carried out on-site. This is not only practical from the point of view of the experiment but in certain cases an absolute necessity from the point of view of the analytics. During the transport and storage of samples (even when using recommended preserving agents) changes in the concentration of the analyte can occur due to conversion reactions (oxidation/ reduction, microbial change) which make it necessary to carry out the test immediately after taking the sample. This is important for the determination of nitrite, the separate recording of Fe(II) and Fe(III) as well as when testing for the detached and easily liberated sulphide, for example.

Summary and Outlook

In summary it can be said that, from the instrumental point of view, due to the numerous innovations regarding the optical configuration, photometry is an instrumental, multifaceted method for quantitative analysis. The instruments on offer of photometric devices gives the user manifold possibilities to analyse liquid samples. The new cuvette geometry and also cuvette free configurations enable the user to test very small volumes of a sample or use pathlength cells, to achieve a very high level of sensitivity. Photometry is a very important supplement to the usual detectors in chromatography and capillary electrophoresis. In the area of environmental analysis, photometry will, without a doubt, keep its high position in the arsenal of analytical methods and probably extend its spectrum of application even further in the future. In my opinion, photometry offers a high potential for environmental analysis in connection with enrichment techniques. The coloured derivates of almost all photometric determining methods can be enriched using solid-phase extraction and after elution can be detected in the solid phase through absorption or reflection measurement. In our work on this topic (we named the method Sorbent Extraction Optosensing, SEOS), we have developed over 20 different procedures for determining environmental analytic relevant parameters and in all cases we have achieved an improvement in the sensitivity of direct photometric determination of one to two orders of magnitude [12].